Metal matrix composites (MMCs) based on magnesium alloys are excellent candidates for engineering light structure materials, and have great potential in automotive, high performance defence and aerospace applications. In spite of relatively high number of papers dealing with microstructure and mechanical properties of MMCs based on Mg alloys, the deformation mechanisms and other physical properties of these materials are not known enough.The objective of this chapter is to reveal influence of various reinforcement types on the mechanical and physical behaviour of composites in which various magnesium alloys were reinforced with short alumina (Saffil®) fibres and/or SiC and Si particles.